Scale is a condition which affects various aspects of the crude oil production, transportation, and refining industry. One particular area which is affected is secondary reservoirs constituted mainly of inorganic chemical compounds, presented in a system which is at least half man-made. Native fluids of a formation and/or changes in the thermodynamic, kinetic and hydrodynamic conditions under which those fluids exist and are produced can cause scale compounds to form.
Scale can reduce formation porosity and permeability when developed on the pores of the formation, especially when this occurs near the well. Scale can also block regular flow when perforations are obstructed or when a thick layer forms on production pipe walls. Scale can also cover and damage completion equipment such as security valves and gas artificial lift system mandrels.
Scale formation begins when any natural fluid condition is disturbed in a way that exceeds the solubility limit of one or more of its components. The first development is generally either a sodden fluid which is made through formation of unstable atom groups (homogeneous nucleation) or a fluid-flow limiting surface which can cause heterogeneous nucleation.
CaCO3 is one of the more common types of scale. Its precipitation depends on CO2 concentration in the system, brine composition, and temperature and pressure control over the chemical equilibrium between CO2 and reservoir formation waters, according to the following reaction:Ca(HCO3)2⇄H2O+CO2+CaCO3 
In fields where water presence is considerable or in those where water injection is used for secondary oil recovery, scale problems can be severe and can increase with time. If this scale is addressed mechanically, the removing methods must be carried out at increasing frequency. Thus, it is generally desirable to prevent scale formation. This can be done as a complementary treatment after mechanical removal of scale.
Existing scale inhibitors have very specific performances, and there is no universal inhibitor for all scale types. Previously, inhibitor efficiency was evaluated by trial and error. Currently, a better understanding of the thermodynamic kinetic mechanism of compound crystal growth allows a better evaluation of the scale inhibitor effectiveness.
In use, inhibitors are injected to the location to be treated either continuously or intermittently. According to the phase where they can be injected, inhibitors are classified as either soluble in organic phase, or soluble in aqueous phase.
The phase in which they are injected is very important since it can affect the inhibitor efficiency. In systems with high water cuts, it is preferred to use an inhibitor which is water soluble, because this phase is the cause of scale incrustation and is the phase with greater contact with internal pipe surfaces. This phase is therefore the better phase for transporting the inhibitor to the metallic surfaces.
Most inhibitors developed to avoid scale are soluble in the aqueous phase because this phase is the main cause of deposit formation. Several researchers have dedicated efforts to understand crystal growth mechanisms that form scale to develop a better inhibitor based on understanding the inhibitor-crystal interaction.
Two ways by which the scale inhibitor operates are known. The first mechanism is the adsorption effect, wherein the inhibitor molecules occupy the nucleation sites which are preferred by the crystals. Thus, crystals cannot find active places to adhere to the surface and, therefore, crystal nucleation is not promoted.
Another inhibitor mechanism is based on an adsorption model, that is, a morphologic change that can prevent formation of crystals in the presence of the inhibitor. Depending on the inhibitor characteristics and the nature of the substrate, it is possible that the inhibitor will be adsorbed over the crystalline net, forming complex surfaces or nets which have difficulty remaining and growing in active places.
Scale inhibitors are generally classified as organic and inorganic. The inorganic types include condensed phosphate, such as polymetaphosphates or dimetallic phosphates.
Inorganic phosphates operate on scale formation through the threshold effect. Through this mechanism, it is not necessary to complex all ions in solution because, when carbonates and calcium sulfur crystals begin to be shaped, they precipitate and, at that moment, phosphate ions cover the small nucleating crystals and crystal growth is atrophied. Coating is given because of phosphate ion adsorption in the crystal surface.
One problem with use of polyphosphates is that in a solution, polyphosphates can suffer hydrolysis or reversion to hydrotreated orthophosphates. Hydrotreated orthophosphates react with calcium to form insoluble calcium phosphates. Temperature, pH, concentration, solution qualities, different types of phosphates and the presence of some enzymes all influence the reversion velocity of these inhibitors.
There are four organic compound groups (polyphosphates, polyphosphonates, polycarboxylic and polymeliates) which have a proven chelant effect over the ions which normally form scale, and these compound groups are used in manufacturing scale inhibitors. Couples of these groups typically are used as follows: (i) phosphonate compounds with alkaline base such as polyphosphates and polyphosphonates, and (ii) weak acids such as polycarboxilyc acid and polymeliates.
The organic phosphate compounds are limited by temperature because they can also revert when exposed to high temperatures. Further, phosphonates are not effective in waters having a high content of calcium ions, and should be applied in large doses.
Polymers obtained from carboxylic acids (that is, polyacrylates) are equally used as scale inhibitors. These compounds tend to distort the crystalline structure of the minerals formed, preventing their adhesion to other crystal and/or to metal surfaces. Temperature of use of these compounds is more stable than the phosphates and phosphonates, however, some polymers have limited tolerance to calcium, generally a maximum concentration of about 2,000 ppm, although some are effective at concentration as high as 5,000 ppm.
For an effective inhibition using these compounds, it is required to inject high polymer concentrations. Taking into account the effectiveness of these compounds at high temperatures (where other products cannot work), this treatment has been considered economical in some instances.
Recently, chelant agents are applied, for example ethylenediaminetetra-acetic acid (EDTA), or its sodium salt, have been used in softening water and/or as scale inhibitor. EDTA forms a soluble and stable complex with magnesium, calcium, strontium, barium and other divalent metals, and this prevents scale formation. This kind of inhibitor does not suffer reversion and is stable at high temperatures. However, these inhibitors are also much more expensive than other products.
Thus, conventional scale inhibitors can be summarized as follows:
Inhibitor TypeLimitationsInorganicSuffer hydrolysis and can precipitatepolyphosphatesas calcium phosphates because oftemperature, pH, solution quality,concentration, phosphate type and thepresence of some enzymes.OrganicSuffer hydrolysis with temperature.polyphosphatesNot effective at high calciumconcentrations. Must be applied inhigh doses.Polymers based onLimited calcium tolerance (2,000 ppm)carboxylic acidsalthough some can work atconcentrations higher than 5,000 ppmLarger concentrations are needed.Ethylenediaminetetra-Very expensive.acetic acid (EDTA)
According to what is known about commercial scale inhibitors, the need arises to develop products which reduce the existing limitations and which are flexible in application.
It is the primary object of the present invention to provide a solution to this need.
Other objects and advantages of the present invention will appear herein.